In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
The constantly increasing demand for high data rates in cellular networks requires new approaches. A challenging question for network operators is how to evolve their existing cellular networks so as to meet the requirement for higher data rates. In this respect, a number of approaches are possible: (i) increase the density of their existing macro base stations; (ii) increase the cooperation between macro base stations; or (iii) deploy smaller base stations in areas where high data rates are needed within a macro base stations grid. Concerning the second approach, e.g., increasing the cooperation between macro base stations, see Y. Liang, A. Goldsmith, G. Foschini, R. Valenzuela, D. Chizhik, “Evolution of Base Stations in Cellular Networks: Denser Deployment vs Coordination”, IEEE ICC 2008, Conference, pp. 5, May 2008.
Such a smaller radio base station is also called a “femto radio base station” and/or a “home radio base station” and/or “pico radio base station” and/or “micro radio base station” in some contexts. All such small radio base stations are collectively referred to herein as a micro base station, which is in contrast to a macro cell covered by a macro or standard radio base station.
The last option is referred to in the related literature as a “Heterogeneous Network”, or “Heterogeneous Deployment” and the layer consisting of smaller base stations is termed a “micro”, or “pico” layer.
Building a denser macro base station grid while simultaneously enhancing the cooperation between macro base stations (hence either using options (i) or (ii) above) is a solution that meets the requirement for higher data rates. However, such an approach is not necessarily a cost-efficient option due, e.g., to the costs and delays associated with the installation of macro base stations, especially in urban areas where these costs are significant.
The solution of deploying small base stations within the already existing macro layer grid is an appealing option, since these smaller base stations are anticipated to be more cost-efficient than macro base stations, and their deployment time will be shorter as well. However, such a dense deployment of macro base stations may lead to a significantly higher amount of signaling due to frequent handovers for users moving at high speeds.
In contrast, the macro layer grid of a heterogeneous network (as illustrated in FIG. 19) can serve users moving at high speed, as well as service wider areas where the demand for high data rates is less and the grid comprising smaller base stations in the heterogeneous network can be employed to service areas having a higher density of users requiring high data rates, or “hotspots” as these areas are termed.
One of the main targets of low power nodes is to absorb as many users as possible from the macro layers. This would offload the macro layer and it will allow for higher data rates in both the macro and in the micro layer.
In this respect, several techniques have been discussed and proposed within 3GPP: (i) extending the range of small cells by using cell specific cell selection offsets; and (ii) by increasing the transmission power of low power nodes & by simultaneously setting appropriately the UL power control target power (P0) for the users connected to low power nodes.
By applying any of the above techniques the interference in the downlink control channels increases. However, since downlink control channels in Long Term Evolution (LTE) are transmitted over the whole bandwidth, classical intercell interference (ICIC) mechanisms cannot be applied.
Hence there is a need for evolved ICIC mechanisms especially for the downlink physical layer control channels. The main technique which has been investigated by the Long Term Evolution (LTE) standardization process is to employ “Almost Blank Subframes” (ABS) at the macro layer. Using ABS, the macro layer is muted so as not to create high other cell interference to users that are both connected to low power nodes and are located at the extended range of the low power nodes.
Almost Blank Subframes (ABS) is a technique which does solve the problem of interference generated by the macro layer to users connected to low power nodes and located at the extended range of low power nodes. However, the drawback of the ABS technique is that resources are not fully used at the macro layer. Moreover, for the case in which the macro layer is heavily loaded and the number of users located at the extended range is low, a number of users will have to underutilize their resources so as not to interfere with users located at the extended range of low power nodes. In such a scenario, the capacity of the heterogeneous network could be limited by congestion or lack of capacity on the downlink control channel.